1. Field of the Invention:
The present invention generally relates to a circuit breaker overcurrent tripping device, and more particularly, to a thermal electromagnetic type overcurrent tripping device.
2. Description of the Prior Art:
An overcurrent tripping device for use with a small-sized circuit breaker, with a rated current for example of 30 A or less, is typically classified as one of two types. The first is a thermal electromagnetic type, in which an electric current flowing through the breaker is designed to flow directly in a bimetallic strip. An electromagnet is activated by a large electric current associated with a short-circuit flowing in the bimetallic strip, tripping the breaker at a set current rating. Another type is a complete electromagnetic type, in which an electromagnet is used in combination with an oil dash-pot. The complicated structure of the complete electromagnetic type makes it very expensive. Further, it is difficult to prevent leakage of oil from the dash-pot. As a result, there is very little demand for the complete electromagnetic type overcurrent tripping device.
Well-known examples of the thermal electromagnetic type overcurrent tripping device are given in FIGS. 3 and 4, wherein a shunt circuit is provided in parallel with the bimetallic strip. Referring first to FIG. 3, an insulating casing 1 and an insulating cover 2 enclose: a contact unit 20, an overcurrent tripping device 40 connected through a flexible conductor 3 and a connecting conductor 4 to contact unit 20, and an opening/closing mechanism 50 connected to contact unit 20 to open and close unit 20. The opening/closing mechanism 50 engages the overcurrent tripping device 40. A power source terminal 5 is connected to the contact unit 20, while a load terminal 6 is connected to the overcurrent tripping device 40. Contact unit 20 is comprised of the following elements: a fixed contact 12 having one end from which the power source terminal 5 is extended and another end to which a fixed contact point 11 is attached; a movable contact 14 having one end to which a movable contact point 13 is fixed and another end connected to the flexible conductor 3; a movable contact support 15 having one end pin-connected to a mid-portion of the movable contact 14 and another end fixed to a cross bar 7 through which the poles communicate with each other; a contact spring 16 stretching between the movable contact support 15 and the movable contact 14, for imparting an impinging pressure between the movable contact point 13 and the fixed contact point 11; and an arc extinction chamber 17 surrounding a contact parting region of the fixed and movable contact points 11 and 13. The overcurrent tripping device 40 is comprised of the following elements: a fixed end connected to the connecting conductor 4 and a free end connected through the flexible conductor 21 to the load terminal 6; a fixed iron core 23 disposed to encompass a bimetallic strip 22, the iron core 23 assuming a substantially C-shape in which one side is open; a movable iron core 25, pivotally supported on the side of the opening of the fixed iron core 23, from which a return spring 24 extends to the fixed iron core 23; an engagement screw 26 so engaged at the free end of the bimetallic strip 22 as to be advanceable and retractable; a tripping lever 27 extended to the movable iron core 25; and a tripping mechanism 30 composed of a tripping lever 28 so disposed as to be capable of engaging with the tip of the tripping lever 27 and a latch receiver 29. The opening/closing mechanism 50 is formed chiefly of a toggle link 42 having one end connected by a pin to the movable contact support 15 and the other end connected by a pin to a latch 41 engaging with the latch receiver 29; a handle lever 44 including its pivotally supported head portion into which an operation handle 43 protruding from a window 2a formed in the cover 2 is embedded; and an opening/closing spring 46 stretching between an articulating pin 45 of the toggle link 42 and the handle lever 43.
Based on this construction, the overcurrent tripping device 40 functions in the following manner. When a current slowly approaching the overcurrent rating flows in the bimetallic strip 22 connected in series to the contact unit 20, the resistance of the bimetallic strip 22 generates Joule heat in the bimetallic strip 22. As a result, the free end of the bimetallic strip 22 is bent counter-clockwise, whereby the tip of the engagement screw 26 causes the tripping lever 28 of the tripping mechanism 30 to rotate counter-clockwise. Then, the engagement of the latch 41 of the opening/closing mechanism 50 with the latch receiver 29 is released, and the toggle link 42 is shifted, thereby causing the contact unit 20 to effect the breaking operation. If a large short circuit accident current flows in the contact unit 20 and the bimetallic strip 22, the bimetallic strip again emits the heat and is bent. In advance of this process, however, a magnetic flux generated in the fixed iron core 23 attracts the movable iron core 25, overcoming a spring force of the return spring 24. As in the case of the slowly increasing overcurrent, the tip of the tripping lever 27 causes the contact unit 20 to effect the breaking operation through the tripping mechanism 30 and the opening/closing mechanism 50.
FIG. 4 depicts another prior art device. In FIG. 4, a shunt circuit is provided consisting of a connecting wire 33 and a contact point unit consisting of a movable contact point 31 and a fixed contact point 32 in parallel with a circuit formed of the bimetallic strip between the connecting plate 4 and the load terminal 6. Note that the movable contact point 31 is fixed to the movable iron core 25, while the fixed contact point 32 is fixed to the fixed iron core 23. The movable contact point 31 is connected through the flexible conductor 34 to the connecting plate 4. In this device, the short circuit accident current is shunted, and the electric current passing through the bimetallic strip 22 is reduced.
In the above-described prior art device, however, if a large short circuit accident current flows as in FIG. 3, an amount of heat generated in the bimetallic strip 22 may exceed an allowable range, and the bimetallic strip 22 can be deformed or melted before performing the breaking process. In the example shown in FIG. 4, if an overcurrent flows in the bimetallic strip 22, the fixed iron core 23 is magnetized by the magnetic flux generated by the electric current, and the movable iron core 25 is attracted into contact with the fixed iron core. Thus, the above-mentioned shunt circuit is formed. However, the generated magnetic flux is frequently too small to attract the movable iron core 25 quickly enough. In the case of the short circuit accident current, the large current flows in the bimetallic strip before the shunt circuit can be formed, so that the bimetallic strip 22 can be deformed or melted, preventing the breaking operation from occurring.